Process for producing propylene based polymer compositions

ABSTRACT

A process for the production of a propylene based polymer, the process comprising the following steps: (a) a first polymerization stage comprising homopolymerizing propylene or copolymerizing propylene and at least one alpha-olefin in the presence of an alpha-olefin polymerization catalyst whereby to produce a polypropylene component; (b) a second polymerization stage comprising copolymerizing ethylene and at least one alpha-olefin in the presence of an alpha-olefin polymerization catalyst whereby to produce an ethylene/alpha-olefin copolymer component; and (c) blending the polymer components produced in steps (a) and (b) whereby to produce a polymer blend, wherein the first and second polymerization stages are effected in separate polymerization reactors connected in parallel. Also provided are polymer compositions comprising: (i) 30 to 97% by weight, based on the total weight of the polymer composition, of a propylene based polymer; and (ii) 3 to 70% by weight, preferably 5 to 20% by weight, based on the total weight of the polymer composition, of an ethylene copolymer plastomer (e.g., an ethylene-propylene plastomer) containing at least 60% by weight ethylene.

FIELD OF THE INVENTION

The present invention relates to improvements in and relating to thepreparation of polypropylene based polymers, in particular to thepreparation of polypropylene polymers having excellent impact strengthas well as high resistance to stress whitening.

DISCUSSION OF THE BACKGROUND ART

Polypropylene has unique properties such as low density, excellentchemical resistance and rigidity. However, certain polypropylenepolymers, e.g. homopolypropylene and polypropylene random copolymers(RACOs), have poor impact resistance especially at low temperatures.This has led to the development of a large number of propylene basedpolymers in which polypropylene polymers are modified by blending withelastomers, e.g. ethylene-propylene rubbers, in which the elastomerforms a dispersed phase in a polypropylene matrix thereby improvingimpact strength. Heterophasic polypropylene copolymers (HECOs), i.e.polymers containing a propylene polymer matrix and an elastomer, are oneexample of such materials.

Although having good impact resistance, heterophasic polypropylenecopolymers are susceptible to stress whitening when exposed tomechanical stresses. When damaged (e.g. when bent or subjected toimpact) the optical appearance of the polymer alters, i.e. this becomesopaque. Although stress whitening may have no effect on the geometricaland/or mechanical properties of the polymer, this nevertheless limitsthe use of such materials in cases where the appearance of the polymerproduct is important, e.g. in the production of toys, household andtechnical appliances, transport and storage boxes, etc. Since stresswhitening can also lead to surface damage, it is also generallyundesirable to use such polymer products for packaging of food ormedical products where it is important that packaging should be keptsterile.

Stress whitening associated with heterophasic polypropylene copolymerscan be reduced by further inclusion within the polypropylene material ofa plastomer. Typically, a plastomer may be dispersed in thepolypropylene material as the result of a blending process.

When blending any polypropylene based polymer (e.g. a polypropylenehomo- or copolymer, or a heterophasic polypropylene copolymer) with anelastomeric polymer (e.g. a plastomer), it is generally considerednecessary to ensure that the density, weight average molecular weight(Mw) and/or MFR of the elastomeric component (e.g the plastomer) ismatched to that of the polypropylene component to ensure adequatehomogenization of the resulting blend. This may, for example, beachieved by prior blending of the polypropylene component with asuitable plastomer.

Typically, polypropylene blends are prepared by blending or compoundingof separate polymer components, e.g. a propylene polymer material and aplastomer, produced in different polymer plants. As a result, transportand handling costs are high. An additional compounding or extrusion stepis also necessary to produce the final polymer blend.

In most polymer plants (e.g. those producing polypropylene),polymerization reactors, supporting systems and extruders are designedto have identical production capacities. This may present problems whenan attempt is made to add a second polymer component (e.g. a plastomer)immediately prior to extrusion—due to the limited capacity of theextruder it is generally necessary to reduce the rate of polymerproduction within the plant. Clearly, this is undesirable.

Alternatively, so-called “reactor blends” can be produced by means of acascade polymerization process in which the same or different catalystsystems are employed to produce different polymers, typically in two ormore separate reactors connected in series. Multi-stage processes inwhich different catalyst systems are employed in sequentialpolymerization stages are described, for example, in EP-A-763553(Mitsui) and WO 96/02583 (Montell). In a cascade process in whichdifferent catalysts are used in sequential reactors, the catalyst fromthe preceding reactor remains active following discharge of the reactionmixture into the next reactor. Inevitably, this results in a lack ofcontrol over the characteristics of the final polymer product. Forexample, in the case where a Ziegler-Natta catalyst used in a firstpolymerization stage remains active during a second stage effected inthe presence of a different catalyst system, a large proportion of thefinal polymer material will comprise a high molecular weight polymerhaving a broad molecular weight distribution. This can lead toundesirable polymer properties.

WO 96/11218 (Montell) describes a multi-stage polymerization process inwhich a first catalyst is deactivated prior to the introduction of asecond catalyst system. Specifically, the process described in WO96/11218 comprises a first stage in which a propylene polymer isproduced in the presence of a first titanium or vanadium catalyst, asecond stage in which the catalyst is deactivated, and a third stage inwhich polymerization is continued in the presence of a secondmetallocene catalyst. Such a cascade process is believed to result ingood homogenization of the resulting polymer blend. However, the need todeactivate the first catalyst before the polymer particles can beimpregnated with the second catalyst makes this process unnecessarilycomplex and not cost effective. A further disadvantage of this processis that the second catalyst is relatively quickly flushed out of thereactor as a result of the high throughput of polymer material into thethird stage of the polymerization process.

Contrary to current thinking, we have now found that the demands of thestep of homogenization of a polypropylene based polymer and anelastomeric polymer, e.g. a plastomer, are not essential to provide apolypropylene material having the desired properties of high impactresistance, resistance to stress whitening, etc. As a result,preparation of the individual polymer materials can be effected inseparate polymerization reactors run in parallel followed by simpleblending (e.g. compounding) of the resulting polymer components. Thisoffers significant advantages in terms of costs, process operability,optimization of desired polymer properties, etc. Surprisingly, we havefound that adequate homogenization can readily be achieved byco-extrusion of the separately produced polymer materials.

SUMMARY OF THE INVENTION

Thus, viewed from one aspect, the invention provides a process for theproduction of a propylene based polymer, which process comprises:

(a) a first polymerization stage comprising homopolymerizing propyleneor copolymerizing propylene and at least one α-olefin in the presence ofan α-olefin polymerization catalyst whereby to produce a polypropylenecomponent;

(b) a second polymerization stage comprising copolymerizing ethylene andat least one α-olefin in the presence of an α-olefin polymerizationcatalyst whereby to produce an ethylene/α-olefin copolymer component;and

(c) blending the polymer components produced in steps (a) and (b)whereby to produce a polymer blend,

wherein said first and second polymerization stages are effected inseparate polymerization reactors connected in parallel.

Preferably, blending may be effected by co-extrusion of the polymercomponents produced in steps (a) and (b). Alternatively, the process ofthe invention may comprise the further step of extruding the polymermixture following simple blending of the polymer components whereby toproduce a substantially homogenous polymer.

In addition to the advantages outlined above, the process hereindescribed effectively permits an increase in the duration of the secondpolymerization stage (since the total output from that stage is theelastomeric, e.g. plastomeric, component). The ability to increase theresidence time in the reactor (e.g. a gas phase reactor) increases theproductivity of any catalyst used. This also permits increasedflexibility of the process in terms of altering the desired mechanicalproperties of the final polymer blend.

Polymers produced by a process according to the invention exhibit goodimpact properties, enhanced stress whitening resistance, and goodoptical properties (haze). Such polymers form a further aspect of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the process flow diagramaccording to the present invention;

FIGS. 2-5 are various graphs which depict the mechanical properties ofthe polymer blends relative to that of the basic polypropylene material;

FIG. 6 is a graph demonstrating that stress whitening resistance isincreased when plastomer is added to the basic polypropylene material;and

FIG. 7 is a graph depicting the analysis results for the products fromExample 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As used herein, the term “polymerization” is not intended to be limitedto homopolymerization, but also includes copolymerization (which termincludes polymerization of two or more comonomers). Similarly, the term“polymer” is not limited to homopolymer but also encompasses copolymer(which term includes polymers derived from two or more comonomers).Since monomer feedstock may comprise small quantities of copolymerizableimpurities, the term “homopolymer” as used herein is intended to denotea polymer deriving at least 99% by weight from a single monomer.

The propylene component that may be prepared in step (a) may be apropylene homopolymer, copolymer or mixture thereof (e.g. a mixture ofcopolymers). Where a copolymer component is produced this may be arandom or heterophasic copolymer. Preferably where a copolymer isproduced this will be a random copolymer. Comonomers which may be usedto produce the copolymers include monomers copolymerizable withpropylene, for example comonomers selected from ethylene and C₄₋₂₀ monoor multiply unsaturated monomers, in particular ethylene and C₄₋₁₀α-olefins, e.g. but-1-ene, pent-1-ene, 3-methyl-but-1-ene,4-methyl-pent-1-ene, hex-1-ene, 3,4-dimethyl-but-1-ene, hept-1-ene,3-methyl-hex-1-ene. Preferably the monomers will be selected fromethene, but-1-ene, hex-1-ene and oct-1-ene. The propylene copolymerswill typically have a propylene content of at least 90 mole %, e.g. atleast 95 mole %.

Where a heterophasic copolymer is produced in the first polymerizationstage, step (a) will typically comprise the following steps:

(i) homopolymerizing propylene or copolymerizing propylene and at leastone α-olefin in a first reactor whereby to produce a polypropylenepolymer or copolymer; and

(ii) further polymerizing propylene and at least one α-olefin (e.g.ethylene) in a further reactor (e.g. a gas phase reactor) in thepresence of said propylene polymer or copolymer whereby to produce aheterophasic copolymer (i.e. said polypropylene component).

Propylene polymers and copolymers produced in the first polymerizationstage will typically have a molecular weight distribution, M_(w)/M_(n),in the range 2.5 to 10, preferably 3 to 8 and a Melt Flow Rate (MFR) at230° C. of 0.1 to 300 dg/min, preferably 0.2 to 150 dg/min, e.g. 1 to 50dg/min. Suitable MFRs may be achieved by control of the degree ofpolymerization of the propylene monomer using techniques well known inthe art. The density of the propylene polymers or copolymers (in theabsence of fillers) will generally range from 890 to 915 g/cm³,preferably from 900 to 915 g/cm³, e.g. 895 to 910 g/cm³.

Where a propylene homopolymer or random copolymer component is producedthis may have an isotactic index of at least 90% measured by NMR.Crystalline or semi-crystalline homopolymers or copolymers of propylenemay be produced in the first polymerization stage. Preferred propylenepolymers include those having a crystallinity greater than 30%,preferably greater than 50%

The ethylene copolymer prepared in step (b) will preferably be anethylene/α-olefin elastomeric polymer, e.g. a plastomer. As used herein,the term “plastomer” is intended to define a class of low densityethylene based copolymers, e.g. having a density of about 870 to 920g/cm³, preferably 880 to 910 g/cm³, at a weight average molecular weight(Mw) of 50,000 to 500,000, preferably 70,000 to 300,000.

Typically, the ethylene copolymer will comprise ethylene and a C₃₋₂₀mono or multiply unsaturated monomer, in particular a C₃₋₁₀ α-olefin,e.g. propene or but-1-ene. Copolymers of ethylene and propylene,especially preferably ethylene propylene elastomeric polymers, arepreferred. Typically, in cases where the comonomer is propylene, theethylene copolymer will have an ethylene content of at least 75%, e.g.at least 85%, by weight.

Ethylene copolymers produced in the second polymerization stage willtypically have a molecular weight distribution, M_(w)/M_(n), in therange 1.5 to 5, preferably 2 to 3 and a Melt Flow Rate (MFR) at 190° C.of 0.01 to 100 dg/min, preferably 0.1 to 50 dg/min, e.g. 0.5 to 30dg/min.

The preparation of polymers having the desired properties for use in theinvention may be achieved using techniques well known in the art, e.g.by appropriate selection of catalyst systems, comonomers, polymerizationreactor type and polymerization process conditions.

Preferably the polypropylene polymer is produced using a conventionalZiegler-Natta catalyst, e.g. a group 4 metal halide, especially a group4 metal chloride such as TiCl₄ or TiCl₃ supported on a suitable carrier.This is advantageously used in the form of spherical particles having amean diameter in the range of from 1 to 200 μm. Suitable methods for thepreparation of such materials are described in the patent and scientificliterature.

Ziegler-Natta catalysts suitable for use in the invention may alsocomprise an electron-donor compound. Examples of electron-donorcompounds include ethers, esters, amines, aldehydes, ketones, alcohols,phenols, carboxylic acids, alkoxy silanes, alkyl alkoxy silanes, etc.

Conveniently the Ziegler-Natta catalyst will be supported using catalystsupports well known in the art, e.g. MgO, SiO₂, magnesium halides suchas MgCl₂, organic polymers such as styrene/divinylbenzene copolymer. Ofthese, MgCl₂ is preferred.

Preferably the polyethylene is produced using a single site catalyst,preferably a single site catalyst capable of substantially homogenousincorporation of comonomer over the MWD of the polymer, in particular ametallocene catalyst. As used herein, the term “metallocene” is used torefer to all catalytically active metal:η-ligand complexes in which ametal is complexed by one, two or more η-ligands. The use of twinη-ligand metallocenes and single η-ligand “half metallocenes” isparticularly preferred. The metal in such complexes is preferably agroup 4, 5, 6, 7 or 8 metal or a lanthanide or actinide, especially agroup 4, 5 or 6 metal, particularly Zr, Hf or Ti. The η-ligandpreferably comprises a cyclopentadienyl ring, optionally with a ringcarbon replaced by a heteroatom (e.g. N, B, S or P), optionallysubstituted by pendant or fused ring substituents and optionally linkedby bridge (e.g. a 1 to 4 atom bridge such as (CH₂)₂, C(CH₃)₂ orSi(CH₃)₂) to a further optionally substituted homo or heterocycliccyclopentadienyl ring. The ring substituents may for example be haloatoms or alkyl groups optionally with carbons replaced by heteroatomssuch as O, N and Si, especially Si and O and optionally substituted bymono or polycyclic groups such as phenyl or naphthyl groups. Examples ofsuch homo or heterocyclic cyclopentadienyl ligands are well known in theart (see e.g. EP-A-416815, WO96/04290, EP-A-485821, EP-A-485823, U.S.Pat. Nos. 5,276,208 and 5,145,819).

Thus the η-bonding ligand may for example be of formula I

CpY_(m)  (I)

where Cp is an unsubstituted, mono-substituted or polysubstituted homoor heterocyclic cyclopentadienyl, indenyl, tetrahydroindenyl, fluorenyl,benzindenyl, cyclopenta[1]phenanthrenyl, azulenyl, or octahydrofluorenylligand; m is zero or an integer having a value of 1, 2, 3, 4 or 5; andwhere present each Y which may be the same or different is a substituentattached to the cyclopentadienyl ring moiety of Cp and selected fromhalogen atoms, and alkyl, alkenyl, aryl, aralkyl, alkoxy, alkylthio,alkylamino, (alkyl)₂P, alkylsilyloxy, alkylgermyloxy, acyl and acyloxygroups or one Y comprises an atom or group providing an atom chaincomprising 1 to 4 atoms selected from C, O, S, N, Si and P, especially Cand Si (e.g. an ethylene group) to a second unsubstituted,mono-substituted or polysubstituted homo or heterocycliccyclopentadienyl, indenyl, tetrahydroindenyl, fluorenyl oroctahydrofluorenyl ligand group.

In the η-bonding ligands of formula I, the rings fused to the homo orhetero cyclopentadienyl rings may themselves be optionally substitutede.g. by halogen atoms or groups containing 1 to 10 carbon atoms.

Many examples of such η-bonding ligands and their synthesis are knownfrom the literature, see for example: Möhring et al. J. Organomet. Chem479:1-29 (1994), Brintzinger et al. Angew. Chem. Int. Ed. Engl.34:1143-1170 (1995).

Examples of suitable i-bonding ligands include the following:

cyclopentadienyl, indenyl, fluorenyl, pentamethyl-cyclopentadienyl,methyl-cyclopentadienyl, 1,3-di-methyl-cyclopentadienyl,i-propyl-cyclopentadienyl, 1,3-di-i-propyl-cyclopentadienyl,n-butyl-cyclopentadienyl, 1,3-di-n-butyl-cyclopentadienyl,t-butyl-cyclopentadienyl, 1,3-di-t-butyl-cyclopentadienyl,trimethylsilyl-cyclopentadienyl, 1,3-di-trimethylsilyl-cyclopentadienyl,benzyl-cyclopentadienyl, 1,3-di-benzyl-cyclopentadienyl,phenyl-cyclopentadienyl, 1,3-di-phenyl-cyclopentadienyl,naphthyl-cyclopentadienyl, 1,3-di-naphthyl-cyclopentadienyl,1-methyl-indenyl, 1,3,4-tri-methyl-cyclopentadienyl, 1-i-propyl-indenyl,1,3,4-tri-i-propyl-cyclopentadienyl, 1-n-butyl-indenyl,1,3,4-tri-n-butyl-cyclopentadienyl, 1-t-butyl-indenyl,1,3,4-tri-t-butyl-cyclopentadienyl, 1-trimethylsilyl-indenyl,1,3,4-tri-trimethylsilyl-cyclopentadienyl, 1-benzyl-indenyl,1,3,4-tri-benzyl-cyclopentadienyl, 1-phenyl-indenyl,1,3,4-tri-phenyl-cyclopentadienyl, 1-naphthyl-indeny,1,3,4-tri-naphthyl-cyclopentadienyl, 1,4-di-methyl-indenyl,1,4-di-i-propyl-indenyl, 1,4-di-n-butyl-indenyl, 1,4-di-t-butyl-indenyl,1,4-di-trimethylsilyl-indenyl, 1,4-di-benzyl-indenyl,1,4-di-phenyl-indenyl, 1,4-di-naphthyl-indenyl, methyl-fluorenyl,i-propyl-fluorenyl, n-butyl-fluorenyl, t-butyl-fluorenyl,trimethylsilyl-fluorenyl, benzyl-fluorenyl, phenyl-fluorenyl,naphthyl-fluorenyl, 5,8-di-methyl-fluorenyl, 5,8-di-i-propyl-fluorenyl,5,8-di-n-butyl-fluorenyl, 5,8-di-t-butyl-fluorenyl,5,8-di-trimethylsilyl-fluorenyl, 5,8-di-benzyl-fluorenyl,5,8-di-phenyl-fluorenyl and 5,8-di-naphthyl-fluorenyl.

Besides the η-ligand, the metallocene catalyst for use in the inventionmay include other ligands; typically these may be halide, hydride,alkyl, aryl, alkoxy, aryloxy, amide, carbamide or other two electrondonor groups.

Particularly preferably the metallocene is an unbridged bis-substitutedcyclopentadienyl zirconium compound in which the substituents, which maybe the same or different, are selected from chloride, amide and methyl.

The catalyst systems herein described may be used in combination withco-catalysts or catalyst activators and in this regard any appropriateco-catalyst or catalyst activator may be used. For metallocenecatalysts, aluminium alkyl compounds, e.g. aluminoxanes, andboron-containing co-catalysts are preferred. Suitable aluminoxanesinclude C₁₋₁₀ alkyl aluminoxanes, e.g. methyl aluminoxane (MAO) andisobutyl aluminoxane, especially MAO.

Aluminoxane co-catalysts are described by Hoechst in WO-A-94/28034.These are considered multicyclic oligomers having up to 40, preferably 3to 20, —[Al(R″)O]— repeat units (where R″ is hydrogen, C₁₋₁₀ alkyl,preferably methyl, or C₆₋₁₈ aryl or mixtures thereof).

The Ziegler-Natta catalyst is preferably activated by trialkyl aluminiumcompounds, e.g. triethyl aluminium.

It is particularly desirable that the metallocene complex be supportedon a solid substrate. Such substrates are preferably porousparticulates, e.g. inorganic oxides such as silica, alumina,silica-alumina or zirconia, inorganic halides such as magnesiumchloride, or porous polymer particles, e.g. acrylate polymer particlesor styrene-divinylbenzene polymer particles which optionally carryfunctional groups such as hydroxy, carboxyl etc. Particle sizes arepreferably in the range 10 to 60 μm and porosities are preferably in therange 1 to 3 mL/g. The complex may be loaded onto the support before, ormore preferably after it has been reacted with a co-catalyst. Desirably,inorganic supports are heat treated (calcined) before being loaded withthe complex.

The process of the invention is carried out in at least twopolymerization stages, preferably using at least two polymerizationreactors connected in parallel and which are operated separately. Eachpolymerization stage may consist of one or more polymerization reactorsand may be effected using conventional procedures, e.g. as a slurry, gasphase, solution or high pressure polymerization. Slurry polymerization(e.g. bulk polymerization) is preferably effected, e.g. in a tankreactor or more preferably a loop reactor.

Each polymerization stage may itself comprise the use of a series of twoor more reactors in which polymerization may be effected under differentconditions. Preferably, each stage will comprise the use of loop and/orgas phase reactors, e.g. a combination of loop and loop, gas phase andgas phase, or most preferably a combination of loop and gas phasereactors. Such multi-stage polymerization processes carried out in twoor more reactors provide for the possibility of independently varying,in any given reactor, process parameters such as temperature, pressure,type and concentration of monomer, concentration of hydrogen, etc. Thisallows a greater degree of control over the composition and propertiesof the resulting polymer material compared with a single-stage process.Any multi-stage process will typically be carried out using the samecatalyst in each reactor—the product obtained in one reactor isdischarged and passed directly into the next reactor without alteringthe nature of the catalyst.

Typical reaction conditions for loop and gas phase reactors are:loop—temperature 60-110° C., pressure 30-100 bar, mean residence time30-120 minutes; and gas phase—temperature 60-110° C., pressure 10-50bar, mean residence time 20-300 minutes. Where hydrogen is used tocontrol molecular weight/MFR₂, the hydrogen partial pressure willtypically be 0.001 to 10 bar.

Preferably, the first polymerization stage used to prepare thepolypropylene matrix will be a multi-stage process in which propylene ishomopolymerized or copolymerized in a series of loop and gas phasereactors. Most preferably, the same catalyst will be used in eachreactor. In the first stage, polymerization of propylene is preferablycarried out at a temperature of 40 to 130° C., preferably 60 to 100° C.and at a pressure of 5 to 100 bar, e.g. 10 to 70 bar.

The second polymerization stage in which the elastomeric component, e.g.a plastomer, is prepared may similarly comprise a multi-stage process.However, typically this will be effected in a single gas phase reactor.In the second stage, copolymerization of ethylene is preferably carriedout at a temperature of 0 to 140° C., preferably 50 to 100° C. and at apressure of 3 to 100 bar, e.g. 5 to 70 bar.

In any of the reactors herein described, the (major) monomer may alsofunction as a solvent/carrier as well as a reagent, or alternatively anon-polymerizable organic compound, e.g. a C₃₋₁₀ alkane, for examplepropane or isobutane, may be used as a solvent/carrier. Where this isdone, the volatile non-reacted or non-reactive materials will desirablybe recovered and re-used, especially where gas phase reactors are used.

A flow diagram illustrating a preferred embodiment of a process inaccordance with the invention is shown in attached FIG. 1. The firststage of polymerization in which propylene is polymerized, optionally inthe presence of one or more α-olefins, comprises sequentialpolymerization in two loop reactors (1, 2) followed by polymerization ina gas phase reactor (3). The catalyst and monomers are fed into thefirst loop reactor (1). Solid polymer material is separated fromnon-reacted material in separator (4) and re-cycled. Preparation of thepolyethylene plastomer is effected in a single gas phase reactor (5)provided with a separate catalyst feed (6). The resulting polymercomponents are then subjected to blending.

The polymer components produced in step (a) and step (b) will typicallybe in the form of a powder, e.g. having a mean particle size from a fewμm up to several mm, preferably from 10 μm to 10 mm, e.g. from 100 μm to6 mm. These components can be compounded or blended together, forexample using conventional extruders in which the individual componentsare blended as a melt under high shear stresses or using conventionalmixing equipment. Alternatively, the components may be blended prior tothe step of melt blending or these can be introduced separately into themelt blending stage.

Single screw extruders, co-rotating or counter-rotating twin-screwextruders, cone extruders, etc. are suitable for use in the invention,for example those available from Werner & Pfleider (W & P), Germany orJapan Steel Work (JSW), Japan. Typically, these will have a barrellength:diameter of about 18 and comprise co-rotating twin-screws eachhaving a screw diameter of about 100 to 400 mm. In general, extrusionmay be effected at a temperature of from 200 to 300° C., at a screwspeed of 200 to 600 rpm, e.g. about 400 rpm, and at a specific energyinput in the range of from 100 to 300 kWh per 1000 kg polymer.

In general, the amount of ethylene copolymer incorporated into the finalpolymer blend will be from 2 to 40% by weight of the composition, e.g. 5to 20% by weight.

The process of the invention can be used to prepare a wide range ofpolymer materials, in particular heterophasic copolymers of propylene.Particularly preferred polymer materials are those formed from apolypropylene based polymer and a plastomer containing more than about60% by weight ethylene. Such materials are considered to be novel andform a further aspect of the invention.

Viewed from a further aspect the invention thus provides a polymercomposition comprising:

(i) 30 to 97% by weight, preferably at least 80%, e.g. 80 to 95% byweight, based on the total weight of the polymer composition, of apropylene based polymer, e.g. a propylene homopolymer, a randomcopolymer of propylene or a polypropylene heterophasic copolymer; and

(ii) 3 to 70% by weight, based on the total weight of the polymercomposition, of an ethylene copolymer plastomer containing at least 60%by weight ethylene.

Particularly preferred compositions in accordance with the inventioninclude those comprising:

(i) 30 to 97% by weight of a propylene based polymer containing greaterthan 95 mol. % propylene units; and

(ii) 3 to 70% by weight of a plastomer containing greater than 75 mol. %ethylene units.

Plastomers comprising ethylene and a C₃₋₁₀ α-olefin, e.g. propene,but-1-ene or octene, are preferred. Most preferably, the plastomercomponent will comprise an ethylene:propylene plastomer.

Typically, the plastomer component will be present in an amount of up to30% by weight, preferably from 2 to 25%, more preferably from 5 to 20%,e.g. 10 to 15% by weight. Because the presence of the plastomer in thepropylene polymer compositions affects the impact strength, the stresswhitening resistance and the stiffness of the product, the specificamount included will depend on the desired balance between theseproperties.

Component (ii) may, for example, comprise an ethylene copolymerplastomer having a substantially uniform or homogenous incorporation ofcomonomer (e.g. propene) over the MWD of the polymer. Most preferably,the plastomer component will consist essentially of propylene andethylene units, i.e. this will be substantially free from unreactedcomonomer material.

Most preferably, the major component of the plastomer present in thecompositions described herein will be ethylene. Typically, this will bean ethylene-propylene plastomer comprising at least 60% by weight,preferably at least 70% by weight, e.g. at least 80% by weight ethylene.The density of the plastomer component will generally be less than 930kg/m³ and will typically lie in the range of from 880 to 920 kg/m³,preferably 890 to 910 kg/m³, e.g. about 902 kg/m³. High melt flow rateethylene copolymer plastomers are preferred, e.g. those having anMFR₂@190° C. in the range of from 0.1 to 60 g/10 min, e.g. 0.5 to 30g/10 min. Such plastomers with butene, hexene or octene as comonomer arecommercially available (e.g. as Exact 0201, Exact 2M011, Exact 4041,Exact 3017, Exact 2M048, Exact 0203, Exact 2M009 and Exact 2M005 fromDex plastomers of the Netherlands). Propylene based plastomers may bemade using polymerization processes known in the art.

In the compositions of the invention the propylene based polymer may beselected from the group consisting of propylene homopolymers, propyleneheterophasic copolymers and propylene random copolymers. Preferred foruse in the invention are propylene based materials having a MFR₂@230° C.of from 0.1 to 200 g/10 min, e.g. from 0.3 to 150 g/10 min.

To ensure satisfactory blending of the propylene based polymer andplastomer, it may be necessary to select materials having similar orsubstantially identical viscosities. For example, the ratio of the MFR₂values for the propylene and plastomer components may be selected to bewithin the range of from 0.1 to 10, e.g. from 0.3 to 5.

Propylene based polymers and plastomer components suitable for use inpreparing the compositions of the invention are known in the art or maybe produced using techniques well known in the art, e.g. by appropriateselection of catalyst systems, comonomers, polymerization reactor typeand polymerization process conditions. Typically, the propylene basedpolymer will be produced using a Ziegler-Natta catalyst, preferably asupported Ziegler-Natta catalyst system (especially a high yield ZieglerNatta catalyst containing Ti, Cl, Mg and Al). The plastomer component ispreferably produced using a metallocene or other single site catalystmaterial, optionally in combination with a co-catalyst.Metallocene:aluminoxane catalyst systems are preferred. The separatepolymer components may be prepared in separate polymerization stagescarried out in separate polymerization reactors connected in parallel asherein described.

The polymer materials described herein have several beneficialproperties, including high transparency and impact strength especiallyat low temperatures, resistance to stress whitening, low levels ofextractables, etc. In particular, these have a good balance betweenimpact resistance, stiffness and stress whitening resistance. Forexample, these may have the following properties:

MFR₂: 0.1 to 200 dg/min, preferably 0.3 to 150 dg/min; Tensile modulus:from 200 to 2,500 MPa, preferably 400 to 2000 MPa;

Impact Falling Weight (Total penetration energy) at 0° C. (measuredaccording to ISO 6603-2): at least 1 J, preferably at least 5 J, e.g. 4to 60 J;

Impact Falling Weight (Total penetration energy) at −20° C. (measuredaccording to ISO 6603-2): at least 0.5 J, preferably at least 4 J;

Charpy impact at 0° C. (measured according to ISO 179/1 eA): 2 to 15KJ/m²;

Charpy impact at −20° C. (measured according to ISO 179/1 eA): 1 to 15KJ/m²;

Stress Whitening Resistance (measured according to BTM 16114, BorealisA/S, Denmark—see Bakshi et al.,

Polymer Testing 8:191-199, 1989, and Example 5 herein): 0 to 10,preferably 0.1 to 5;

Haze (measured according to ASTM 1003 on 2 mm injection moulded plates):less than 70%, preferably less than 40%.

The polymers produced in accordance with the invention may be formulatedtogether with conventional additives, e.g. antioxidants, UV-stabilizers,colors, fillers, etc. prior to use. Alternatively, such additives may beadded to the polypropylene/plastomer mixture prior to blending.

The resulting polymer materials can be readily extruded, molded orotherwise formed to produce a wide range of articles, such ascontainers, boxes, crates, toys, household articles, automotive parts,cups, films, profiles, pipes, cable insulation, etc. Generally, thepolymer pellets formed following extrusion are supplied to an injectionmolding machine for injection molding into shaped products. Films mayalso be produced using conventional cast film techniques andsubsequently processed by thermoforming. Alternatively, the polymerpellets may be blow moulded using conventional moulding equipment, orsubjected to further grinding to form micropellets or powder which maybe used in rotomolding techniques.

Viewed from a yet further aspect the invention provides the use of apolymer composition according to the invention in the manufacture ofarticles having high impact resistance, for example by a molding orextrusion technique.

Due to their low level of extractables, the polymers herein describedmay also find use in packaging of food and medical products where it isessential that this should not contaminate the packaged product.

The invention will now be further described with reference to thefollowing non-limiting Examples and the accompanying Figures in which:

FIG. 1 is a schematic representation of a parallel reactor process inaccordance with the invention;

FIG. 2 is a chart showing various mechanical properties of aheterophasic polypropylene copolymer (MFR₂@230° C.=3.5 g/10 min) blendedwith C3 and C8 plastomers (values given relative to the heterophasiccopolymer). BC250P (commercially available from Borealis A/S, Denmark)is a reference polymer material having a higher content ofethylene-propylene rubber than the heterophasic polypropylene copolymer;

FIG. 3 is a chart showing various mechanical properties of a randompolypropylene copolymer (MFR₂@230° C.=1.5 g/10 min) blended with C3 andC8 plastomers (values given relative to the random copolymer);

FIG. 4 is a chart showing various mechanical properties of a randompolypropylene copolymer (MFR₂@230° C.=12 g/10 min) blended with C3 andC8 plastomers (values given relative to the random copolymer);

FIG. 5 is a chart showing various mechanical properties of apolypropylene homopolymer (MFR₂@230° C.=2.0 g/10 min) blended with C3and C8 plastomers (values given relative to the polypropylenehomopolymer).

FIG. 6 is a chart showing various mechanical properties of aheterophasic polypropylene copolymer (MFR₂@230° C.=13 g/10 min) blendedwith C4 and C8 plastomers (Modulus values are multiplied by 100; stresswhitening resistance (SW) values are multiplied by 0.1); and

FIG. 7 is a chart showing haze, falling impact weight (Tot. en) andCharpy impact properties for a random polypropylene copolymer blendedwith various C8 plastomers.

EXAMPLE 1 Preparation of Polymer Materials

A propylene based polymer powder is produced using a conventionalZiegler-Natta catalyst in a first reactor or series (cascade) ofreactors. The product may be a homopolymer, a random co-polymer orheterophasic co-polymer.

In a second gas phase reactor a single site catalyst is used to producean ethylene based polymer powder. Density and MFR of the resultingpolymer may be controlled by comonomer and hydrogen concentrations. Thereaction temperature is controlled to achieve optimum catalytic activityand to avoid fouling of the reactor. The reactor temperature may be aslow as 70° C.

When the desired density and MFR are achieved for the polyethylenepolymer, this is combined with the polypropylene polymer produced in thefirst reactor, preferably in a deactivating unit (e.g. a steamer such asconventionally used in the Spheripol process). The combined product maybe treated at this stage, e.g. by adding conventional additives such asanti-oxidants, nucleators, fillers, anti-static agents, etc., prior tocompounding in a conventional extruder.

It is to be expected that for a given reactor volume of the second gasphase reactor, the residence time will be relatively long compared to aconventional cascade process (e.g. as described in WO 96/11218 toMontell). For example, in the case where the ethylene based polymercomprises 10% by weight of the total polymer produced, the residencetime in the second gas phase reactor may be expected to be 10 timeslonger than in a conventional cascade process. As a result, the catalystproductivity (i.e. kg polymer produced per g catalyst) can be expectedto be significantly higher when using the process in accordance with theinvention.

EXAMPLE 2 Compounding Process

Polypropylene random copolymers produced using Ziegler-Natta catalystsare co-extruded with single site polyethylene copolymers produced usingoctene and butene. Extrusion may be effected using a conventionaltwin-screw extruder (e.g. available from Werner & Pfleider (W & P),Germany operated at a temperature of from 200 to 300° C., a screw speedof about 400 rpm, and at a specific energy input in the range of from100 to 300 kWh per 1000 kg polymer.

EXAMPLE 3 Preparation of a C3 Plastomer

A total of 950 g C3 plastomer (Samples 1-10) was produced using a GasPhase reactor (bench scale). Each polymerization reaction was effectedusing the catalyst bis-indenyl-zirconium-di-chloride (supported onsilica) with MAO as a co-catalyst under the following reactionconditions:

Starting temperature: 60° C. Polymerization temperature: 70° C. Amountof propylene: 15 g (31.7 mol %) Amount of ethylene: 68.3 mol %Polymerization pressure: 1.64 MPa Amount of catalyst: 105-115 mg

In each case, 0.2% anti-oxidant (Irganox 1010 from Ciba) was added tothe final polymer powder. The anti-oxidant was added as a solution inmethanol and the solvent was subsequently evaporated at ambienttemperature.

The properties of Samples 1-10 are shown in Table I below:

TABLE I ¹MFR₂ ²MFR₂₁ MFR₂₁/ Density Yield Sample (g/10 min) (g/10 min)MFR₂ (kg/m³) (g) 1 0.53 8.6 16.2 907.0 91 2 0.49 8.3 16.9 905.9 90 30.47 7.7 16.4 907.3 91 4 0.57 9.4 16.7 907.0 92 5 0.52 8.6 16.5 905.7 906 0.50 8.1 16.2 906.9 92 7 0.53 8.6 16.2 907.1 90 8 0.57 9.4 16.5 907.094 9 0.57 9.4 16.5 905.8 91 10 0.53 8.9 16.8 906.6 91 ¹MFR₂ determinedat 190° C. using 2.16 kg load according to ISO 1133 ²MFR₂₁ determined at190° C. using 21.6 kg load according to ISO 1133

Samples 1-10 were subsequently compounded in a 16 mm Prism extruder(available from PRISM, Staffordshire, UK). MFR₂₁ of the final product at230° C. was 1.0 g/10 min.

EXAMPLE 4 Preparation of Polymer Blends With C3 and C8 Plastomers

Various polymer materials were obtained as set out in Table II below.Polymer blends were prepared by compounding the basic polypropylenematerial with the plastomer using a 16 mm Prism extruder.

TABLE II Product Polypropylene (PP) PE No. material Plastomer Additives1 100 wt. % BC240P¹ — — 2  85 wt. % BC240P¹ 15 wt. % C8 — plastomer⁵ 3 85 wt .% BC240P¹ 15 wt. % C3 — plastomer⁶ 4 100 wt. % CHC3007² — — 5 85 wt. % CHC3007² 15 wt. % C8 — plastomer⁵ 6  85 wt. % CHC3007² 15 wt.% C3 — plastomer⁶ 7 100 wt. % RE420MO³ — — 8  85 wt. % RE420MO³ 15 wt. %C8 — plastomer⁵ 9  85 wt. % PE420MO³ 15 wt. % C3 — plastomer⁶ 10 100 wt.% DS10⁴ — 2000 ppm MdBS⁷ 11  85 wt. % DS10⁴ 15 wt. % C8 2000 ppmplastomer⁵ MdBS⁷ 12  85 wt. % DS10⁴ 15 wt. % C3 2000 ppm plastomer⁶MdBS⁷ ¹BC240P = heterophasic polypropylene co-polymer available fromBorealis A/S, Denmark (MFR₂₁ @ 230° C. = 3.5 g/10 min, Tensile modulus =1418 MPa) ²CHC3007 = random polypropylene copolymer available fromBorealis A/S, Denmark (MFR₂₁ @ 230° C. = 1.5 g/10 min, Tensile modulus =792 MPa) ³RE420MO = random polypropylene copolymer available fromBorealis A/S, Denmark (MFR₂₁ @ 230° C. = 12.0 g/10 min, Tensile modulus= 884 MPa) ⁴DS10 = polypropylene homopolymer available from BorealisA/S, Denmark (MFR₂₁ @ 230° C. = 2.0 g/10 min, Tensile modulus = 1521MPa) ⁵C8 plastomer (Exact 0201 available from Dex-Plastomers − comonomer= octene, MFR₂₁ @ 190° C. = 1.1 g/10 min, density = 902 kg/m³). ⁶C3plastomer produced in accordance with Example 3 ⁷1,3-di(methylbenzylidene) sorbitol (CAS No. 54686-97-4)

Specimens were analysed to determine the following properties:

Falling Weight Impact (FWI) determined according to DIN53443

Optical Properties (Haze) measured for 2 mm injection moulded platesaccording to ASTM D1003

Tensile Modulus determined according to ISO 572-2

Charpy notched impact determined according to ISO 179/1 eA

Mechanical properties of the polymer blends relative to that of thebasic polypropylene material are shown in attached FIGS. 2 to 5.

FIG. 2 shows that by adding 15 wt. % C8 plastomer, FWI@ −20° C. can beincreased to a level of 4.5 times that of the basic polypropylenematerial (BC240P). This is roughly equivalent to that of the referencematerial BC250P (available from Borealis A/S, Denmark). By adding 15 wt.% C3 plastomer, the increase in FWI@ −20° C. is to a level about 2.5times that of BC240P. An increase in the amount of plastomer in theblend could result in even higher FWI and thus increased stiffnessproperties, should this be desirable. Although stress whiteningresistance (SWR) was not measured for the blended materials, this can beexpected to be at least equal or better than that for the base polymer,BC240P. The increase in impact resistance (Charpy) at −20° C. is 15% forthe C3 plastomer blend, 30% for the C8 plastomer blend and 60% forBC250P. An increase in impact strength (Charpy) could similarly beexpected with an increase in the amount of plastomer.

As shown in FIG. 3, both C3 and C8 plastomers give a dramaticimprovement in FWI@ 0° C. This is of particular interest when using thepolymer materials at refrigeration temperatures. Both plastomers makethe deformation ductile and the increase in FWI@ 0° C. is about 40 timesthat for the basic polypropylene material (RACO). The effect is marginalat −20° C. At +23° C., a significant increase in impact strength(Charpy) is seen for both C3 (+70%) and for C8 (+90%) plastomer blends.Charpy was not measured at 0° C. At −20° C. the increase in Charpy forC3 is +10% and for C8 is +40%. The increase in Haze seen for the C3plastomer compared to the C8 plastomer is believed to be caused by thehigher density and possibly the broad co-monomer distribution of the C3plastomer. For a C3 plastomer made under constant reactor conditions andhaving a target density of 902 kg/m³, the haze could be expected to beidentical to that of the C8 plastomer. The C8 plastomer gives a veryacceptable Haze.

In FIG. 4, FWI at −20° C. is increased 10-fold for the C3 plastomer and20-fold for the C8 plastomer compared to the basic polypropylenematerial (RACO). At −20° C. the increase is +80% for both plastomers. At+23° C. both C3 and C8 plastomers result in an approx. 2-fold increasein impact resistance (Charpy). At −20° C. Charpy is unchanged by theaddition of plastomer. It can be concluded that both C3 and C8plastomers give a positive contribution to impact resistance.

In FIG. 5, impact-stiffness balance is favourable for the blendedmaterials at normal temperatures.

Conclusions

RACO—at refrigeration temperatures, 15 wt. % C3 plastomer can givesignificantly improved impact behaviour without destroying the good hazevalues of the base polymer. A density in the range of from 890 to 910kg/m³, preferably about 902 kg/m³, is considered optimal.

HECO—although C3 plastomers are not as efficient as higher olefins, theimprovement in impact resistance is nevertheless significant. It isbelieved that a reduction in the density of the C3 plastomer or anincrease in the amount of C3 plastomer would result in an increase inCharpy and FWI@ −200° C. It can be concluded that C3 plastomers areeffective as impact modifiers in heterophasic polypropylene materials.

EXAMPLE 5 Mechanical Properties of Polymer Blends With C4 and C8Plastomers

Various polymer materials were prepared as set out in Table III below.Polymer blends were prepared by compounding the polypropylene materialand plastomer in a conventional compounding extruder (Prism TSE24HC,twin screw, available from PRISM, Staffordshire, UK).

TABLE III Product Polypropylene (PP) No. material PE Plastomer 1 100 wt.% BE222P¹ — 2  85 wt. % BE222P¹ 15 wt. % C8 plastomer² 3  85 wt. %BE222P¹ 15 wt. % C4 plastomer³ 4  85 wt. % BE222P¹ 15 wt. % C4plastomer⁴ ¹BE222P = heterophasic polypropylene copolymer available fromBorealis A/S, Denmark (MFR₂ @ 230° C. = 13, Tensile Modulus (accordingto ISO 572-2) = 1500 MPa, Charpy notched impact (according to ISO179/1eA) @ 23° C. = 6.5 kJ/m² and @ 0° C. = 4 kJ/m², IFW (according toISO 6603-2) @ 0° C. = 27 J, @ 20° C. = 20 J) ²C8 plastomer (Exact 2M011available from Dex-Plastomers − MFR ₂ @ 190° C. = 1.1 g/10 min, density= 882 kg/m³) ³C4 plastomer (Exact 4041 available from Dex-Plastomers −MFR ₂ @ 190° C. = 2.6 g/10 min, density = 882 kg/m³) ⁴C4 plastotner(Exact 3017 available from Dex-Plastomers − MFR₂ @ 190° C. = 27 g/10min, density = 901 kg/m ³)

The products were analysed to determine various mechanical propertiesand the results are shown in attached FIG. 6.

Stress Whitening Resistance (SW) was determined according to thefollowing method:

Definitions:

Diameter is measured on “whitened” area in two directions:

1. From point of injection through the center point

2. Perpendicular to the first line

Degree of whitening is defined as delta L (DL), CIE Lab Difference(difference between unstressed and stressed material).

Method:

Circular Plates are injection moulded (diameter=60 mm, thickness=2 mm,5.66 wt. % blue masterbatch (Voor 78000304) is added—blue articles arevery sensitive to Stress Whitening). Plates are stressed by a Fallingweight. Testing is done at ambient temperature. The Falling weight isequipped with an auto-catcher to prevent rebouncing at the testedsample.

Apparatus Used:

Hammel falling apparatus, type BSM 4000/1700 VD (available from HammelMaskinfabrik A/S, Denmark).

Spectrophotometer: ICS Texion

Soft-ware: Datacolor International-dataMaster v2.0

Sample conditioning is 7 days at ambient temperature before stressing.Measurement of Stress Whitening is done one day after stressing.

Mass of weight = 4.0 kg Diameter of dart = 20 mm (half sphere shaped)Diameter of hole where the sample rests = 25 mm Height of fall = 100 mmTemperature = 23° C.

Color measurement is determined according to CIE Lab difference. Delta L(DL) is the ratio of the lightening in color of unstressed and stressedplates. A higher value for delta L represents an increase in stresswhitening.

Measurement is done on the side of the plate opposite to that which ishit by the ball (diamater of aperture=25 mm). Delta L is measured atD65/10°.

From FIG. 6 it can be seen that stress whitening resistance (SWR) isincreased when plastomer is added to the basic polypropylene material.C4 plastomer is more effective in improving SWR than C8 plastomer. InIFW, the influence of the MFR and density of the plastomer dominates theeffect of the comonomer. The C8 plastomer is found to be most effectivein increasing Charpy values.

EXAMPLE 6 Properties of Polymer Blends With C8 Plastomers

Various polymer materials were prepared as set out in Table IV below.Polymer blends were prepared by compounding the polypropylene materialand plastomer in a 24 mm Prism extruder.

TABLE IV Product Polypropylene (PP) No. material PE Plastomer 1 100 wt.% RE222P¹ — 2  90 wt. % RE222P¹ 10 wt. % C8 plastomer² 3  90 wt. %RE222P¹ 10 wt. % C8 plastomer³ 4  90 wt. % RE222P¹ 10 wt. % C8plastomer⁴ 5  90 wt. % RE222P¹ 10 wt. % C8 plastomer⁵ ¹RE222P = randompolypropylene copolymer available from Borealis A/S, Denmark (MFR₂ @230° C. = 13 g/10 min) ²C8 plastomer (Exact 2M048 available fromDex-Plastomers − MFR₂ @ 190° C. = 10 g/10 min, density 902 kg/m³) ³C8plastomer (Exact 0203 available from Dex-Plastomers − MFR₂ @ 190° C. 3g/10 min, density = 902 kg/m³) ⁴C8 plastomer (Exact 2M009 available fromDex-Plastomers − MFR₂ @ 190° C. = 10 g/10 min, density = 882 kg/m³) ⁵C8plastomer (Exact 2M055 available from Dex-Plastomers − MFR₂ @ 190° C. 3g/10 min, density = 882 kg/m³)

The products were analysed to determine various properties and theresults are shown in attached FIG. 7.

The density of 902 kg/m³ is considered to be close to the optimaldensity for desired impact modification of polypropylene RACOs withretained haze values.

What is claimed is:
 1. A process for the production of a propylene basedpolymer, which process comprises: (a) a first polymerization stagecomprising homopolymerizing propylene or copolymerizing propylene and anα-olefin in the presence of an α-olefin polymerization catalyst wherebyto produce a polypropylene component; (b) a second polymerization stagecomprising copolymerizing ethylene and an α-olefin in the presence of anα-olefin polymerization catalyst whereby to produce an ethylene/α-olefincopolymer component; and (c) blending the polymer components produced insteps (a) and (b) whereby to produce a polymer blend, wherein said firstand second polymerization stages are effected in separate polymerizationreactors connected in parallel.
 2. A process as claimed in claim 1wherein the polymer components produced in step (a) and step (b) areeach in the form of a powder.
 3. A process as claimed in claim 1,wherein the polymer components produced in step (a) and step (b) are fedinto a single deactivating unit.
 4. A process as claimed in claim 1,wherein step (c) is effected by co-extrusion of said polymer components.5. A process as claimed in claim 1, wherein said first polymerizationstage is effected in the presence of a Ziegler-Natta catalyst.
 6. Aprocess as claimed in claim 1, wherein said second polymerization stageis effected in the presence of a metallocene or other single-sitecatalyst.
 7. A process as claimed in claim 1, wherein said firstpolymerization stage is a multi-stage process in which propylene ishomopolymerized or copolymerized in a series of loop and/or gas phasereactors.
 8. A process as claimed in claim 1, wherein said secondpolymerization stage is effected in a single gas phase reactor or in twoor more gas phase reactors.
 9. A polymer material produced by a processas claimed in claim 1.